Re: [Qemu-devel] [RFC v4 24/71] s390x: convert to cpu_halted

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 31 Oct 2018 16:56:54 +0000
Alex Bennée <alex.bennee@linaro.org> wrote:

> Emilio G. Cota <cota@braap.org> writes:
> 
> > On Wed, Oct 31, 2018 at 16:13:05 +0000, Alex Bennée wrote:  
> >> > @@ -353,10 +355,12 @@ void s390_cpu_unhalt(S390CPU *cpu)
> >> >      CPUState *cs = CPU(cpu);
> >> >      trace_cpu_unhalt(cs->cpu_index);
> >> >
> >> > -    if (cs->halted) {
> >> > -        cs->halted = 0;
> >> > +    cpu_mutex_lock(cs);
> >> > +    if (cpu_halted(cs)) {
> >> > +        cpu_halted_set(cs, 0);
> >> >          cs->exception_index = -1;
> >> >      }
> >> > +    cpu_mutex_unlock(cs);  
> >>
> >> I think this locking is superfluous as you already added locking when
> >> you introduced the helper.  
> >
> > It gives a small perf gain, since it avoids locking & unlocking twice
> > in a row (cpu_halted and cpu_halted_set both take the lock if needed).  
> 
> Maybe a one line comment then above the first lock:
> 
>   /* lock outside cpu_halted(_set) to avoid bouncing */
> 
> Or some such.....

Yes, please.

Or introduce something like cpu_halted_flip(cs, new_state) where you
could convert the above to

if (cpu_halted_flip(cs, 0)) { /* cpu halted before */
    cs_exception_index = -1;
}

Or is that actually an uncommon pattern?

> 
> >  
> >> Otherwise:
> >>
> >> Reviewed-by: Alex Bennée <alex.bennee@linaro.org>  
> >
> > Thanks!
> >
> > 		Emilio  
> 
> 
> --
> Alex Bennée